Delete SQLAlchemy query using row_number()

Question

I'm looking for an appropriate way to make query.delete() using row_number() function.

Below there is my current solution:

subquery = session.query(Foo,
                         func.row_number().over(order_by=desc(Foo.bar)).label("row_number")).subquery()
subquery = session.query(subquery).filter(subquery.c.row_number > 3)
subquery = subquery.from_self(Foo.id)

query = session.query(Foo).filter(
    Foo.id.in_(subquery)
)

I don't like that I need explicilty check if Foo.id in list of ids. I need to compare all object's columns without hardcoding them. Somthing like:

subquery = session.query(Foo,
                         func.row_number().over(order_by=desc(Foo.bar)).label("row_number")).subquery()
subquery = session.query(subquery).filter((subquery.c.row_number > 3))
subquery = subquery.from_self(Foo)

query = session.query(Foo).filter(
    Foo.in_(subquery)
)

But of course this does not work.

I would be grateful if anyone will suggest a solution.

Answer 1

After some time of investigations, I understood the following:

1. You need to specify exact model fields for filtration. I created a method _get_reference_attrs that uses primary key fields, or all fields, if there is no primary key.
2. Code in question used 3 nested select queries (not including delete query), but only 2 are required. For this you need to correctly collect what you are selecting in the upper selection (attrs_subquery)

That's my solution:

from sqlalchemy import func, desc, tuple_, inspect


def _get_reference_attrs(model):
    """
    Collecting attributes of model that will be used in delete query
    By default the primary key columns are used. If no primary key - all columns are used.
    """

    mapper = inspect(model)
    primary_key_names = [primary_key.key for primary_key in mapper.primary_key]
    if primary_key_names:
        attrs = [attr for attr in mapper.attrs if attr.key in primary_key_names]
    else:
        attrs = mapper.attrs

    return attrs


attrs = _get_reference_attrs(Foo)

subquery = session.query(*attrs,
                         func.row_number().over(order_by=desc(Foo.bar)).label("row_number")).subquery()
attrs_subquery = [getattr(subquery.c, attr.key) for attr in attrs]
subquery = session.query(*attrs_subquery).select_from(subquery).filter(subquery.c.row_number > 1)

query = session.query(Foo).filter(
    tuple_(*attrs).in_(subquery)
)